Came across this thread while trying to find some info on my DT-3. 
My question is: Piston that I removed from motor is stamped 9 75.
Bottom of jug has 0.018 on it. Could someone tell me what I am 
dealing with so I know what piston I should be looking for? I know
now I didn't pay enough attention in math class! Thanks, D

Dennis D...  nobody can tell you what piston you need from the info provided.  Your really need to find a good machine shop that is experienced in these old two strokes and have cylinder measured and likely bored/honed oversize.  Or send it to someone like Bill Bune.

That said, it sounds like piston that is in there now is still standard size.

Thanks Mark. Bike is showing 4xxx miles on it, but they were rough ones
as the bike was used on a farm and not taken care of very well. When I pulled
the jug it had some rtv on the gasket so maybe somewhere it its life it got a
new set of rings. The 75 on the piston made me wonder if it was a 3rd over
piston in it. Then I read this thread and had other thoughts. I don't have a
bore mic. to check it with. I have to take it to my friends machine shop to have
the exhaust bolts removed tomorrow and I will have him look at it for me.